AGRICULTURAL PHOSPHORUS AND WATER-QUALITY - SOURCES, TRANSPORT AND MANAGEMENT

Freshwater eutrophication is usually controlled by inputs of phosphoru s (P). To identify critical sources of P export from agricultural catc hments we investigated hydrological and chemical factors controlling P export from a mixed land use (30% wooded, 50% cultivated, 20% pasture ) 39.5-ha catchment in east-central Pennsylvania, USA. Mehlich-3 extra ctable soil P, determined on a 30-m grid over the catchment, ranged fr om 7 to 788 mg kg(-1). Generally, soils in wooded areas had low Mehlic h-3 P (<30 mg kg(-1)), grazed pasture had Mehlich-3 P values between a nd 200 mg kg(-1), and cropped fields receiving manure and fertiliser a pplications were in most cases above 200 mg kg(-1). Average P concentr ations for ten storms during 1996 decreased 50% downstream from segmen t 4 to segment 1 (catchment outlet). Flow-weighted streamflow P concen trations were more closely related to the near-stream (within 60 m) th an whole catchment distribution of high-P soils. This suggests that ne ar-stream surface runoff and soil P are controlling P export from the catchment. Remedial measures should be targeted to these critical P so urce areas in a catchment. Measures include source (fertiliser and man ure application) and transport management (reduce surface runoff and e rosion).